1. Field of the Invention
The present invention relates to a frequency-characteristic switchable buffer circuit, and particularly to a frequency- characteristic switchable buffer circuit wherein high frequency selective characteristics are respectively added to signals having selectively-supplied first and second frequencies, whereby selected frequency signal components are outputted in a state in which non-selected frequency signal components are sufficiently attenuated.
2. Description of the Related Art
As mobile communication systems employed in all the countries of the world, there are known a DCS (Digital Cellular System) employed in England, Germany, Italy, France and partial countries in Asia, and a GSM (Global System for Mobile Communications) which is the European unified standard system for a digital portable cellular phone, which has been adopted in 1982, and which is used in partial countries in Europe, America, Africa and Asia.
In this case, the DCS is a mobile communication system wherein 1805 MHz through 1880 MHz, 1710 MHz through 1785 MHz and a 1700 MHz band are respectively assigned to a base-station frequency, a mobile-station frequency, and an oscillating frequency generated from a voltage-controlled oscillator (VCO) of a portable cellular phone, the number of channels to be used is 374, and GMSK (Gaussian Minimum Shift Keying) is used as a modulation scheme. On the other hand, the GSM is a mobile communication system wherein 925 MHz through 960 MHz, 880 MHz through 915 MHz and a 900 MHz band are respectively assigned to a base-station frequency, a mobile-station frequency, and an oscillating frequency generated from a voltage-controlled oscillator (VCO) of a portable cellular phone, the number of channels to be used is 124, and the GMSK (Gaussian Minimum Shift Keying) is used as a modulation scheme.
Since these two mobile communication systems, i.e., the DCS and GSM are originally mobile communication systems different in system from each other, two portable cellular phones: a portable cellular phone used when mobile communications are performed by the DCS and a portable cellular phone used when mobile communications are made by the GSM, are needed to enter the two mobile communication systems of DCS and GSM.
Since, however, the DCS and GSM are both GMSK in use modulation scheme and simply different from each other in assignment use frequency as described above, a portable cellular phone capable of being shared between the DCS and GSM has already been proposed wherein two voltage-controlled oscillators: a first voltage-controlled oscillator for oscillating a first frequency lying within a 1700 MHz band and a second voltage-controlled oscillator for oscillating a second frequency lying within a 900 MHz band, and a switching type oscillator having a switching circuit are placed within the portable cellular phone, and the switching circuit of the switching type oscillator is controlled to thereby allow the proper use of the two voltage-controlled oscillators.
In the portable cellular phone capable of being shared between the DCS and GSM, when the mobile communications are performed by the DCS, the switching circuit is used to bring the first voltage-controlled oscillator to an operating state and bring the second A voltage-controlled oscillator to a non-operating state, whereby an oscillating signal having a first frequency is outputted from the first voltage-controlled oscillator.
On the other hand, when the mobile communications are carried out by the GSM, the switching circuit is used to bring the second voltage-controlled oscillator to an operating state and bring the first voltage-controlled oscillator to a non-operating state, whereby an oscillating signal having a second frequency is outputted from the second voltage-controlled oscillator. Further, the oscillating signal having the first frequency or the oscillating signal having the second frequency is selected and amplified by a buffer circuit capable of performing switching between frequency characteristics, followed by supply to an available circuit.
The frequency-characteristic switchable buffer circuit used at this time has a parallel resonance circuit (frequency selection circuit). When the first voltage-controlled oscillator is kept in the operating state or the second voltage-controlled oscillator is kept in the operating state, the parallel resonance circuit is supplied with a first switching voltage or a second switching voltage to thereby select its characteristic selective characteristic. When the first switching voltage is supplied to the parallel resonance circuit, the parallel resonance circuit selects and outputs the oscillating signal having the first frequency- When the second switching voltage is supplied thereto, the parallel resonance circuit selects and outputs the oscillating signal having the second frequency.
FIG. 5 is a circuit diagram showing one example of a configuration of a conventional frequency-characteristic switchable buffer circuit employed in the portable cellular phone capable of being shared between the DCS and GSM.
As shown in FIG. 5, the conventional frequency-characteristic switchable buffer circuit 50 has an amplifying stage with a transistor 51, base bias resistors 55 and 56, an emitter resistor 57, and a bypass capacitor 58; a parallel resonance circuit (frequency selection circuit) 52; coupling capacitors 53 and 54; a frequency switching voltage generator 71 with a one-circuit two-contacts selector switch 59, switching-voltage setting resistors 60 and 61, and a bypass capacitor 62; a buffer resistor 63; a signal input terminal 64; a signal output terminal 65; and a power terminal 66. These circuit elements are electrically connected to one another as shown in FIG. 5.
Further, the parallel resonance circuit 52 comprises a first inductor 521, a second inductor 522, a first capacitor 523, a second capacitor 524, a third capacitor 525, and a switching diode 526. These circuit elements 521 through 526 are electrically connected to one another as shown in FIG. 5.
As shown in FIG. 5, a first voltage-controlled oscillator 67 for oscillating a first frequency, in this case, each of frequencies lying within a 1700 MHz band, a second voltage-controlled oscillator 68 for oscillating a second frequency, in this case, each of frequencies lying within a 900 MHz band, a power supply or battery 69, and a one-circuit two-contacts selector switch 70 are placed on the preceding stage side of the frequency-characteristic switchable buffer circuit 50. These circuit elements 67 through 70 are electrically connected to one another as shown in FIG. 5. In this case, the selector switch 59 and the selector switch 70 are selected or changed in interlock with each other by a control signal to be described later.
The conventional frequency-characteristic switchable buffer circuit 50 having the above-described configuration is operated as follows:
When the portable cellular phone is used in a mobile communication based on the DCS, movable contacts of the selector switch 59 and the selector switch 70 are respectively changed over from positions indicated by solid lines shown in FIG. 5 to positions indicated by dotted lines illustrated in FIG. 5 according to a first control signal outputted from a controller (not shown). Owing to the changeover, the first voltage-controlled oscillator 67 is electrically connected to the power supply 69 so that it is brought to an operating state. Thus, a signal having a first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is outputted from the first voltage-controlled oscillator 67. On the other hand, the second voltage-controlled oscillator 68 is electrically disconnected from the power supply 69 so that it is brought to a non-operating state. Owing to the changeover of the movable contact of the selector switch 59 to the position indicated by the dotted line, a first switching voltage corresponding to a positive voltage is supplied to the parallel resonance circuit 52 so that the parallel resonance circuit 52 parallel-resonates at the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) as will be described below.
When the signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) outputted from the first voltage-controlled oscillator 67 is supplied to the signal input terminal 64 at this time, only the signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is selected and amplified by the parallel resonance circuit 52 which is electrically connected as a load of the transistor 51 and resonates in parallel at the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band), when the signal is amplified by the common emitter-connected transistor 51. Afterwards, the amplified signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is supplied to an available circuit through the signal output terminal 65.
On the other hand, when the portable cellular phone is used in a mobile communication based on the GSM, the movable contacts of the selector switch 59 and the selector switch 70 are respectively changed over to the positions indicated by the solid lines shown in FIG. 5 according to a second control signal outputted from the controller in the same manner as described above. Owing to the changeover, the second voltage-controlled oscillator 68 is electrically connected to the power supply 69 so that it is brought to an operating state. Thus, a signal having a second frequency (any of frequencies lying within a 900 MHz band) is outputted from the second voltage-controlled oscillator 68. On the other hand, the first voltage-controlled oscillator 67 is electrically disconnected from the power supply 69 so that it is brought to a non-operating state. Owing to the changeover of the movable contact of the selector switch 59 to the position indicated by the solid line, a second switching voltage corresponding to a ground voltage is supplied to the parallel resonance circuit 52 so that the parallel resonance circuit 52 parallel-resonates at the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) as will be described below.
When the signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) outputted from the second voltage-controlled oscillator 68 is supplied to the signal input terminal 64, only the signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) is selected and amplified by the parallel resonance circuit 52 which is electrically connected as the load of the transistor 51 and resonates in parallel at the second frequency (corresponding to any of the frequencies lying within the 900 MHz band), when the outputted signal is amplified by the common emitter-connected transistor 51. Afterwards, the amplified signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) is supplied to the available circuit through the signal output terminal 65.
Next, a description will be made of operation at the time that a first switching signal or a second switching signal is supplied to the parallel resonance circuit 52 to thereby change the parallel resonance frequency of the parallel resonance circuit 52 to the first frequency or the second frequency.
When the second switching voltage corresponding to the ground voltage is first supplied to the parallel resonance circuit 52 to cause the parallel resonance circuit 52 to parallel-resonate at the signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band), the second switching signal voltage corresponding to the ground voltage is supplied to the parallel resonance circuit 52. As a result, the switching diode 526 is turned off so that a point where the second capacitor 524 and the third capacitor 525 are connected to each other, is brought to a state of being separated from a ground point. At this time, a parallel resonance frequency approximately equal to the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) is set to the parallel resonance circuit 52 by a first parallel-connected circuit comprising the first inductor 521 and the first capacitor 523, and a second parallel-connected circuit comprising the second inductor 522 series-connected to the first parallel-connected circuit and the second capacitor 524 and the third capacitor 525. Upon such parallel resonance, the second parallel-connected circuit indicates or takes a small capacitor at the second frequency (corresponding to any of the frequencies lying within the 900 MHz band), and the parallel resonance circuit 52 results in an equivalent circuit comprising a series circuit comprised of the first inductor 521 and the small capacitor, and the first capacitor 523 parallel-connected to the first inductor 521. Thus, the parallel resonance frequency of the parallel resonance circuit 52 can be easily set to the second frequency (corresponding to any of the frequencies lying within the 900 MHz band).
Next, when the first switching signal corresponding to the positive voltage is supplied to the parallel resonance circuit 52 to thereby allow the parallel resonance circuit 52 to parallel-resonate at the signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band), the first switching signal corresponding to the positive voltage is supplied to the parallel resonance circuit 52. As a result, the switching diode 526 is turned on so that the connecting point provided between the second capacitor 524 and the third capacitor 525 is brought to a state of being connected to the ground point. At this time, the side at a point where the second inductor 522 is electrically connected to the first inductor 521, is electrically connected to the ground point through the second capacitor 524 and the turned-on switching diode 526, and the first parallel-connected circuit comprised of the first inductor 521 and the first capacitor 523 can be neglected. Therefore, a parallel resonance frequency approximately equal to the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is set to the parallel resonance circuit 52 by the second inductor 522, the second capacitor 524 electrically parallel-connected thereto, and the third capacitor 525. Upon the parallel resonance, the parallel-connected circuit comprising the second capacitor 524 and the third capacitor 525 indicates an integrated capacitor, and the parallel resonance circuit 52 results in an equivalent circuit comprising the second inductor 522 and integrated capacitor connected in parallel. Thus, the parallel resonance frequency of the parallel resonance circuit 52 can be easily set to the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band).
Next, FIG. 6 is a characteristic diagram showing one example of a gain vs. frequency characteristic obtained in the conventional frequency-characteristic switchable buffer circuit 50 shown in FIG. 5.
In FIG. 6, the horizontal axis indicates a frequency (whose unit is represented in GHz), and the vertical axis indicates gain (dB). A curve (indicated by a solid line) a is a characteristic obtained when the buffer circuit 50 parallel-resonates at the first frequency lying in the 1.7 MHz band (1700 MHz band). A curve (indicated by a dotted line) b is a characteristic obtained when the buffer circuit 50 is tuned to the second frequency lying in the 0.9 GHz (900 MHz band).
As indicated by the curves a and b in FIG. 6, when the signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is selected as an amplifying signal, the gain of the buffer circuit 50 with respect to the signal having the first frequency increases, whereas when the signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) is selected as an amplifying signal, the gain thereof with respect to the signal having the second frequency increases.
The conventional frequency-characteristic switchable buffer circuit 50 can take out a signal of a first frequency having large signal amplitude because the gain thereof with respect to the signal having the first frequency is large, when the signal having the first frequency (corresponding to any of frequencies lying within a 1700 MHz band) is selected as an amplifying signal. On the other hand, when a signal having the second frequency (corresponding to any of frequencies lying within a 900 MHz band) is selected as an amplifying signal, the frequency-characteristic switchable buffer circuit 50 can take out the signal of the second frequency having large signal amplitude in the same manner as described above since the gain thereof with respect to the second frequency is large.
However, since the conventional frequency-characteristic switchable buffer circuit 50 does not obtain attenuation with respect to a signal on the side of a frequency higher than the first frequency in a large way so much when the signal having the first frequency (corresponding to any of the frequencies lying within the 1700 MHz band) is selected, the rate of the output of a signal having a spurious frequency relative to the first frequency simultaneously with the signal having the first frequency becomes high. Further, since attenuation of harmonic components of the signal having the second frequency is not obtained in a large way so much when the signal having the second frequency (corresponding to any of the frequencies lying within the 900 MHz band) is selected, the rate of the output of a signal having a spurious frequency simultaneously with the signal having the second frequency increases.
With such a background of art in view, it is therefore an object of the present invention to provide a frequency-characteristic switchable buffer circuit capable of increasing attenuation of a signal having a spurious frequency upon selection of a signal having a first frequency and increasing attenuation of a signal having a spurious frequency upon selection of a signal having a second frequency.
In order to achieve the above object, there is provided a frequency-characteristic switchable buffer circuit according to the present invention, which comprises an amplifying stage, a parallel resonance circuit which serves as an output load of the amplifying stage, a frequency trap circuit connected between an input of the amplifying stage and a reference potential point, and a frequency switching voltage generator for selectively generating a first switching voltage or a second switching voltage, and wherein the parallel resonance circuit parallel-resonates at a first frequency when the first switching voltage is supplied thereto and parallel-resonates at a second frequency different from the first frequency when the second switching voltage is supplied thereto, and the frequency trap circuit comprises a series circuit comprised of a diode and a capacitor, and series-resonates at the second frequency when the first switching voltage is supplied thereto and series-resonates at the first frequency when the second switching voltage is supplied thereto.
As one preferred example having the above configuration, there is also provided a frequency-characteristic switchable buffer circuit wherein a first frequency is supplied from a first oscillator, a second frequency is supplied from a second oscillator and the first oscillator and second oscillator are alternately changed to an operating or non-operating state, and when the first oscillator is in the operating state, a frequency switching voltage generator is switched so as to output a first switching voltage and when the second oscillator is in the operating state, the frequency switching voltage generator is switched so as to output a second switching voltage.
As another preferred example having the above configuration, there is further provided a frequency-characteristic switchable buffer circuit wherein a second frequency and a first frequency have a relationship between a fundamental wave frequency and a harmonic frequency thereof, the second frequency is directly supplied from an oscillator and the first frequency is supplied from a harmonic generator to which the second frequency is applied, and when the first frequency is supplied from the oscillator, a frequency switching voltage generator is changed so as to output a first switching voltage and when the second frequency is supplied from the harmonic generator, the frequency switching voltage generator is changed so as to output a second switching voltage.
According to the above configuration, when a first switching voltage is supplied to a parallel resonance circuit to allow the parallel resonance circuit to parallel-resonate at a first frequency, thereby selecting and outputting a signal having the first frequency, the first switching voltage is supplied even to a frequency trap circuit to thereby cause the frequency trap circuit to series-resonate at a frequency lower than the first frequency. As a result, a trap is developed on the side of a frequency higher than the first frequency by its bound or bounce. The developed trap serves so as to attenuate signal components of a spurious frequency of the signal having the first frequency. On the other hand, when a second switching voltage is supplied to the parallel resonance circuit to cause the parallel resonance circuit to parallel-resonate at a second frequency, thereby selecting and outputting a signal having the second frequency, the second switching voltage is supplied even to the frequency trap circuit to thereby allow the frequency trap circuit to series-resonate at a frequency in the neighborhood of 1700 MHz, thereby serving so as to attenuate signal components of a spurious frequency of the second frequency. Thus, when the signal having the first frequency is selected, attenuation of signal components of a spurious frequency in the signal having the first frequency can be increased. Further, when the signal having the second frequency is selected, attenuation of a signal having a spurious frequency included in the signal having the second frequency can be increased, whereby the signal having the first frequency or the signal having the second frequency can be selected and outputted in a state in which the spurious signal components are little included therein.